1. Field
The present disclosure relates generally to sensors and, in particular, to a method and apparatus for inspecting infrared sensors. Still more particularly, the present disclosure relates to a method and apparatus for inspecting materials for infrared sensors.
2. Background
Optical detector devices are hardware that respond to light. For example, an infrared focal plane array (FPA) is a type of infrared detector device that generates images using infrared light. These infrared detector devices include materials that generate signals when exposed to infrared light, and circuits that process the signals.
The infrared focal plane array functions by absorbing photons in a material. The absorption of the photons results in signals being generated based on the carriers that are detected in the material. These signals are used to form images.
The photons absorbed in the material cause the formation of carriers in the material. Carriers are electron-hole pairs. For example, when a photon is absorbed in the material, an electron in a valance band in the material may gain energy and jump to a conduction band leaving a hole in the valence band to form a carrier. These carriers may be detected to create signals used to form an image.
A circuit connected to the material detects a voltage caused by the carriers. Based on the detection of the voltage generated by the carriers, signals may be generated by the circuit to form the image.
The carriers have a lifetime during which the carriers are present before recombination occurs. Recombination occurs when an electron fills the hole. For example, collisions between at least two electrons in a conduction band may result in one of the electrons recombining in the valence band. This type of recombination is an auger recombination. Radiative recombination is another type of recombination that involves band to band recombination. The length of time during which a carrier exists before recombination is referred to as a carrier lifetime.
In some cases, recombination may occur before the carriers can be detected and before the signals can be generated. The quality of the material may affect the ability of an optical sensor to generate signals in response to photons or thermal energy. Defects in the material used to detect photons may result in a recombination occurring quick enough such that a signal is not generated or the signal is not generated at a level that accurately reflects the presence of the carriers. This type of recombination is a Shockley-Read Hall (SRH) recombination, which is also referred to as a trap-assisted recombination.
These inconsistencies in the material may be, for example, a defect, a material impurity, or some other undesired inconsistency. This type of recombination is undesirable because a signal is not generated indicating the detection of a photon. As a result, the infrared detector device may not indicate as many detections of photons as desired.
As a result, signals may be generated that are not caused by the absorption of photons by the material. Consequently, the performance of an infrared detector device may suffer from this type of recombination.
The amount of Shockley-Read Hall recombination that occurs depends on the quality of the material. The quality of the material may vary between infrared detector devices and within an infrared detector device.
The variance of the quality of materials may result in an undesired quality in the images generated by an infrared focal plane array. For example, the inconsistencies in an infrared focal plane array may result in an undesired quality in a pixel or in hundreds of pixels depending on the size and location of the inconsistencies.
Inspections of the material for infrared focal plane arrays are performed. Defects in the material are often not readily identifiable, however, until the infrared focal plane array is fully fabricated and connected to circuits to read signals and generate images.
Testing at this stage of manufacturing for infrared focal plane arrays may be more expensive and time-consuming than desired. For example, if the images generated by an infrared focal plane array do not have a desired level of quality, the infrared focal plane array may be discarded. At this point, the cost and time to connect the circuits to the material has occurred. As a result, new circuits are needed, as well as another piece, or chip, of the material.
The large format of infrared focal plane arrays is very expensive and the production is generally of a low volume, such as 10 devices or less. Thus, it is desirable to know whether the quality of the material is suitable for use in an infrared focal plane array as soon as possible to avoid wasting resources on continuing manufacturing of an infrared focal plane array that has defects in the material for detecting photons.
Therefore, it would be desirable to have a method and apparatus that take into account at least some of the issues discussed above, as well as other possible issues. For example, it would be desirable to have a method and apparatus for inspecting materials at a time in manufacturing that overcome the technical problem of wasting resources and time that occurs when infrared detector devices are currently tested.